Oxide scale induced cleavage fracture in an ODS Fe-Cr-Al alloy
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Oxide Scale Induced Cleavage Fracture in an ODS Fe-Cr-AI Alloy J. M. DAVIDSON, C.M. AUSTIN, and M. L. ROBINSON It is well recognized that bcc metals and alloys are prone to cleavage fracture when deformed at temperatures below their respective ductile-brittle transition temperatures. Although cleavage phenomena are well characterized, several differing models have been proposed to describe the micromechanisms of cleavage. Many earlier theories have related cleavage to deformation induced crystalline flaws such as twin intersections, 1 or dislocation pile-ups at either precipitates, 2 grain boundaries, 2'3 or intersecting slip bands. 4 Models have also been proposed which attribute cleavage fracture to deformation induced microstructural flaws such as cracked carbides, 5 with the possible added component of matrix dislocation pile-ups at these flaws. 6'7 In engineering materials such as ferritic stainless steels, cleavage is most often attributed to cracking in or near precipitates such as carbides, nitrides, oxides, or other deleterious second phases. 8 Recently, a novel mode of cleavage initiation has been observed in an oxide dispersion strengthened (ODS) ferritic stainless steel, INCOLOY* alloy MA 956. Cleavage frac*A trademark of the Inco family of companies.
ture in this alloy can be initiated by the presence of a surface oxide scale developed during elevated temperature exposure. This mechanism appears to be related to complex interactions between the oxide scale and the underlying alloy substrate during post-exposure deformation. Description of the Cleavage Phenomenon. INCOLOY alloy MA 956 is an Fe-20 pct Cr-4.5 pct A1-0.5 pct Ti0.5 pct Y203 (in wt pct) alloy produced by the mechanical alloying process.9"1~The fine Y203 dispersoid, together with the coarse, elongated grain structure developed in this material, provide excellent stress rupture strength in comparison to conventional high temperature alloys at temperatures around 1000 ~ and higher, l~ This alloy also possesses outstanding resistance to high temperature oxidation, carbuJ.M. DAV]DSON, C.M. AUSTIN, and M.L. ROBINSON are all Senior Metallurgists with Inco Alloy Products Company Research Center, Inco-Sterling Forest, E O. Box 200, Suffern, NY 10901. Manuscript submitted October 5, 1982. 1516--VOLUME I4A, JULY 1983
rization, and sulfidation, l~ The present experiments were performed on sheet material, which is the most common commercial product form of this alloy. Under certain conditions, long-term, elevated temperature exposures in air can degrade the room temperature ductility of MA 956 sheet. These exposures are typically of several hundred to several thousand hours duration at temperatures of approximately 1000 to 1200 ~ After such exposures, tensile ductility can be lowered from typical values of 12 to 15 pct elongation for as-received sheet, to 1 to 2 pct elongation. Attendant with the loss of tensile ductility is a degradation in sheet formability as measured by Erichsen cup tests or bend tests. In all cases, cleavage fracture occurs well a
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